Dissolved Oil Removal (DOR) Unit is an additional liquid-liquid extraction process, using aromatic rich hydrocarbon solvent, that may remove reactive dissolved hydrocarbons from pretreated net quench water used in the process of dilution steam generation of steam cracker plants. The step of pretreating may remove dispersed oil from the aqueous phase of the net quench water.
Present technologies for pretreating net quench water may be described as free dispersed oil coalescing units. Some units may include filters followed by a coalescer, Dispersed Oil Extractor (DOX) system, and Induced Gas Floatation (IGF) system. All these units may coalesce dispersed oil droplets in the net quench water and remove the coalesced oil from the net quench water. None of the aforementioned units are capable of removing the dissolved oils that are in the bulk aqueous phase. Some of the hydrocarbons present in the bulk aqueous phase may react in units downstream of the coalescing unit which may cause fouling of dilution a steam generator and a gaseous hydrocarbon steam saturator. A DOR unit may be used to further treat pretreated net quench water to reduce the amount of dissolved hydrocarbons in the net quench water. Reduction or removal of dissolved hydrocarbons may reduce fouling of the dilution steam generator and the gaseous hydrocarbon steam saturator.
Base petrochemicals such as olefins (alkenes) may be produced in steam cracking plants from saturated aliphatic hydrocarbon feedstocks, such as ethane, propane, butanes or higher molecular weight hydrocarbon mixtures such as naphtha, atmospheric and/or vacuum gas oils, and the like. Generally, pressures may be close to atmospheric (e.g., from about 1.5 to 2.5 barg.), and temperatures may be from approximately 700° C. to approximately 870° C. Steam may be added to the hydrocarbon feed to reduce the hydrocarbon partial pressure. Steam-to-hydrocarbon feed ratios may be generally 0.3-0.4:1 on a weight basis for light hydrocarbon feedstocks such as ethane or propane, and butanes, respectively. The saturated hydrocarbon-steam mixture may be thermally cracked to lower molecular weight unsaturated hydrocarbons. Cracking product reactions may include ethylene predominately, followed by propylene, and then various quantities of C4, C5 and C6 mono- and diolefinic hydrocarbons, with a lesser quantity of C7 and higher weight saturated and unsaturated aliphatic, cyclic and aromatic hydrocarbons.
Additionally, the thermal cracking process may produce some molecules that tend to combine to form high molecular weight materials which can be categorized within the boiling range of “fuel oil” and heavier compounds categorized as “tar”. Tar is a high-boiling point, viscous, reactive material that can foul equipment under certain conditions. In general, feedstocks containing higher boiling materials tend to produce greater quantities of tar. Unsaturated hydrocarbons are reactive and may polymerize upon exposure to high temperatures which may cause fouling of equipment.
One reason a steam cracking unit may be using ethane as a feedstock is because ethane is a co-product of natural gas from shale gas production, and has limited value for uses other than as a feedstock to a steam cracker unit. As natural gas demand and production rates grow for supplying electrical power and home heating needs, ethane availability may increase beyond its domestic regional demand. Since ethane cannot be readily or economically transported, regional demand is important and where its availability exceeds regional demand, its price is reduced. In many regions, ethane feed costs may be 25% to 50% of other steam cracker feedstocks such as propane, butanes or naphthas. This economic scenario gives rise to a large advantage to producing ethylene using low cost ethane feedstock. In addition, energy costs and capital investments for a steam cracker using ethane feedstock may be far below the costs for using propane, butanes or naphthas as feedstock(s).
Following thermal cracking of saturated hydrocarbons, the effluent from the pyrolysis reactor must be rapidly cooled to a temperature at which no additional reaction occurs. This rapid cooling may be effected by indirectly cooling the effluent in typical Transfer Line Exchanger(s) (TLE) which generates high pressure steam and then further directly cooled by circulating water created from condensation of steam within a Quench Water Tower.
For gaseous feedstocks (ethane, propane and butanes), a Quench Oil Tower (QOT) may not be required because only small amounts of C5+ liquids may be produced. For these feedstock types, a simple Quench Water Tower (QWT) is used to cool the effluent gas from the TLE.
The cracked gas may be further cooled in the QWT by direct contact with quench water. Typically, the bottoms stream leaving the quench tower feeds an oil-water separator (OW/S) drum, which function as a three-phase separator, with a light hydrocarbon phase that floats on water, and the tar which sinks in water, as the bottom phase, and water as the middle phase. Even in the case of cracking an ethane feed which may have a relatively lower tar yield than other feedstocks, the small amounts of tar may build up and over time and foul downstream units. In particular, water leaving the OW/S may contain enough heavy oils and tar, which has a specific gravity close to that of water, to potentially cause downstream fouling of the quench circuit. This can also potentially result in the fouling of downstream heat exchangers and water stripping towers, which, when fouled, must be taken offline for cleaning.
The gross Quench Water (QW), from the OW/S, may contain residual fine solid particles, unsettled free oil, emulsified oil, and dissolved hydrocarbons. The majority of this gross QW may be recirculated for low-level heat recovery within the ethylene plant before returning to the QWT. The net raw QW may be either: (1) used to generate dilution steam for steam cracking as a close-loop system, or (2) purged to battery limits as an open-loop system. The net QW may be processed to remove the residual suspended solids, as well as free and emulsified oil, in order to prevent or reduce fouling in a downstream dilution steam generation system. Alternatively, if the excess raw water were simply purged to battery limits, it would still be necessary to remove organic impurities (e.g. benzene, dienes, and other carcinogens) to such an extent that it could be discharged into local streams without causing pollution.
At present, the net QW is treated by coalescing its free and dispersed oil using any combination of the available coalescing technologies including: filter-coalescer, Natco DOX (Dispersed Oil Extraction) unit, and IGF (Induced Gas Floatation). These technologies are effective in removing most of the free and dispersed oil but remain incapable of removing the dissolved unsaturated hydrocarbons. Any remaining dissolved unsaturated hydrocarbons in the QW may polymerize leading to fouling of the steam generation equipment. Additionally, discharges of net QW and blowdowns from dilution steam generators and gaseous hydrocarbon saturators may have difficulties in meeting the environmentally required oil and benzene content.